There are numerous applications where it is necessary to convert variable speed motive power into constant frequency electrical power for one or more AC loads. In the past, this has been accomplished in aircraft applications through the use of a hydromechanical constant speed drive which is coupled to the engine of the aircraft and which converts the variable speed motive power produced by the engine into constant speed motive power. A synchronous generator is coupled to the output of the constant speed drive and converts the constant speed motive power into constant frequency AC power for the loads. Such a generating system is sometimes referred to as an integrated drive generator (IDG).
More recently, attempts have been made at designing a practical generating system which converts variable speed motive power into constant frequency electrical power without the use of hydromechanical components in the constant speed drive. For example, Dishner et al. U.S. Pat. No. 4,695,776 and Borger U.S. Pat. No. 4,572,961 disclose electrically compensated constant speed drives which include a mechanical differential having a first input coupled to the output of the prime mover and an output coupled to a synchronous generator. A speed compensation link in the form of first and second permanent magnet machines interconnected by a power converter is coupled between the prime mover and a second input of the differential. The power flow between the permanent magnet generators is controlled so that the required compensating speed is provided to the second differential input shaft to maintain the output shaft at a constant speed.
An alternative to the foregoing systems that does not use a constant speed drive is referred to as a variable speed, constant frequency (VSCF) generating system which includes a synchronous generator coupled directly to the prime mover and a power converter which converts the variable frequency output of the generator into constant frequency power for the loads. Interest in VSCF systems has increased of late owing to the push to design "all electric" aircraft in which the use of mechanical, hydromechanical and hydraulic components is minimized.
Typically, the output voltage of the VSCF system at a point of regulation (POR) is controlled by controlling the excitation of the synchronous generator. For example, Baker U.S. Pat. No. 4,554,501 discloses a VSCF system which operates within a normal generator speed range to regulate inverter output voltage for AC loads by controlling exciter field current. During auxiliary operation when the speed of the generator is below the normal design speed range such that power cannot be supplied to AC loads, a DC voltage supplied to the inverter on a DC link is regulated at a desired level by controlling the exciter field current and the DC link voltage is provided to DC loads. Thus, DC link regulation is effective only during auxiliary operation when the speed of the generator is insufficient for the inverter to supply AC power to AC loads. In neither mode of operation does the system regulate inverter output voltage by controlling the inverter.
Glennon U.S. Pat. No. 4,527,226 discloses a VSCF control which determines a link value representing what the level of the DC link voltage should be based upon the load at the POR. The actual DC link voltage is measured and is divided by the link value to obtain a value PUV.sub.DC which is used to select a switch control pattern from a memory. The switch control pattern is used to control switches in the VSCF inverter. This control, however, does not attempt to control DC link voltage to in turn determine PUV.sub.DC in a way which minimizes distortion in the inverter output.